LU-5929 tests: conf-sanity test 72 call tune2fs on MDSs

The call to tune2fs tuning MDTs with "-O extents" is now
run on the MDS node(s) and not the client.

Test-Parameters: alwaysuploadlogs testlist=conf-sanity

Signed-off-by: James Nunez <james.a.nunez@intel.com>
Change-Id: Ie4e1bb2d9447d86c9b0144e8f2564b5c2444842d
Reviewed-on: http://review.whamcloud.com/12761
Tested-by: Jenkins
Reviewed-by: Jian Yu <jian.yu@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>